TS1091: Only a single variable declaration is allowed in a 'for...in' statement

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Understanding TS1091: Only a single variable declaration is allowed in a 'for...in' statement

The error TS1091 arises in TypeScript when you attempt to declare multiple variables within a for...in loop. A for...in loop iterates over the enumerable properties of an object. However, TypeScript enforces that only a single variable declaration is permitted within this loop.

An Example of the Error

Consider the following code snippet that will trigger the TS1091 error:

let obj = { a: 1, b: 2, c: 3 };

for (let x in obj, y in obj) { // This will cause TS1091
    console.log(x, y);
}

In this code, we're trying to declare x and y together in a single for...in statement, which is not allowed. The error TS1091: Only a single variable declaration is allowed in a 'for...in' statement will be thrown.

How to Fix It

To resolve this issue, you need to declare one variable in the for...in loop, like this:

let obj = { a: 1, b: 2, c: 3 };

for (let x in obj) {
    console.log(x, obj[x]); // Accessing the property value correctly
}

Now, we have a single variable x iterating over obj. You can access the corresponding value using obj[x], which is how you typically extract values from an object in TypeScript.

Important Things to Know about TS1091

• Single Declaration: Remember, only one variable can be declared in a for...in loop.
• Variable Scope: The variable declared in the loop (x in our case) is scoped to that loop.
• Property Access: You can access object properties using bracket notation within the loop.
• Type Safety: TypeScript will enforce type checks on the variable declared, which can help catch potential errors early.
• Use Cases: The for...in loop is particularly useful when you need to iterate over object properties where the order is not essential.

FAQ about TS1091

Q: What happens if I try to declare multiple variables?
A: TypeScript will throw an error (TS1091) and refuse to compile the code.

Q: Can I use for...of instead?
A: Yes, for...of can be used for iterable objects (like arrays) and allows only one variable declaration too.

Q: Why does TypeScript enforce this rule?
A: This rule helps maintain code clarity and reduces the chances of confusing logic errors.

By adhering to these guidelines, you can avoid the TS1091: Only a single variable declaration is allowed in a 'for...in' statement error in your TypeScript code. This will improve your coding practices and lead to cleaner, more maintainable code.